xarray builds upon two of the foundational libraries of the scientific Python
stack, NumPy and pandas. It is written in pure Python (no C or Cython
extensions), which makes it easy to develop and extend. Instead, we push
compiled code to optional dependencies.

The core internal data structure in xarray is the Variable,
which is used as the basic building block behind xarray’s
Dataset and DataArray types. A
Variable consists of:

dims: A tuple of dimension names.

data: The N-dimensional array (typically, a NumPy or Dask array) storing
the Variable’s data. It must have the same number of dimensions as the length
of dims.

attrs: An ordered dictionary of metadata associated with this array. By
convention, xarray’s built-in operations never use this metadata.

encoding: Another ordered dictionary used to store information about how
these variable’s data is represented on disk. See Reading encoded data for more
details.

Variable has an interface similar to NumPy arrays, but extended to make use
of named dimensions. For example, it uses dim in preference to an axis
argument for methods like mean, and supports Broadcasting by dimension name.

However, unlike Dataset and DataArray, the basic Variable does not
include coordinate labels along each axis.

Variable is public API, but because of its incomplete support for labeled
data, it is mostly intended for advanced uses, such as in xarray itself or for
writing new backends. You can access the variable objects that correspond to
xarray objects via the (readonly) Dataset.variables and
DataArray.variable attributes.

xarray is designed as a general purpose library, and hence tries to avoid
including overly domain specific functionality. But inevitably, the need for more
domain specific logic arises.

One standard solution to this problem is to subclass Dataset and/or DataArray to
add domain specific functionality. However, inheritance is not very robust. It’s
easy to inadvertently use internal APIs when subclassing, which means that your
code may break when xarray upgrades. Furthermore, many builtin methods will
only return native xarray objects.

The standard advice is to use composition over inheritance, but
reimplementing an API as large as xarray’s on your own objects can be an onerous
task, even if most methods are only forwarding to xarray implementations.

If you simply want the ability to call a function with the syntax of a
method call, then the builtin pipe() method (copied
from pandas) may suffice.

To resolve this issue for more complex cases, xarray has the
register_dataset_accessor() and
register_dataarray_accessor() decorators for adding custom
“accessors” on xarray objects. Here’s how you might use these decorators to
write a custom “geo” accessor implementing a geography specific extension to
xarray:

importxarrayasxr@xr.register_dataset_accessor('geo')classGeoAccessor(object):def__init__(self,xarray_obj):self._obj=xarray_objself._center=None@propertydefcenter(self):"""Return the geographic center point of this dataset."""ifself._centerisNone:# we can use a cache on our accessor objects, because accessors# themselves are cached on instances that access them.lon=self._obj.latitudelat=self._obj.longitudeself._center=(float(lon.mean()),float(lat.mean()))returnself._centerdefplot(self):"""Plot data on a map."""return'plotting!'

This achieves the same result as if the Dataset class had a cached property
defined that returns an instance of your class:

classDataset:...@propertydefgeo(self)returnGeoAccessor(self)

However, using the register accessor decorators is preferable to simply adding
your own ad-hoc property (i.e., Dataset.geo=property(...)), for several
reasons:

It ensures that the name of your property does not accidentally conflict with
any other attributes or methods (including other accessors).

Instances of accessor object will be cached on the xarray object that creates
them. This means you can save state on them (e.g., to cache computed
properties).

Using an accessor provides an implicit namespace for your custom
functionality that clearly identifies it as separate from built-in xarray
methods.